Many injection molding systems use a sprue bushing to provide a transfer melt channel between the machine nozzle and a distribution manifold in a mold. In the event that the machine nozzle has to be disengaged from making contact with the sprue bushing, molten plastic will drool backwards from the sprue bushing, and this will adversely affect the next shot of melt. This situation can happen in many applications, such as for example with shuttle molds, rotary molds, and stack molds.
Due to residual pressures in the system, molten material tends to leak, or “drool”, from the sprue bushing at the end of the injection cycle, i.e., when the machine nozzle is retracted from the sprue bushing inlet. This drool represents waste material and increased production costs. Further, the drooled material may collect on the mold and prevent complete closing thereof or cause permanent damage thereto.
Many techniques and devices exist in the art that are designed to prevent or reduce drool in an injection molding system. Examples include various shut-off mechanisms located at the machine nozzle tip or spindle elements for use in a “suck-back” procedure. In many cases, the machine nozzle incorporates an anti-drool mechanism into its structure, as is described in U.S. Pat. No. 3,934,626 to Hall, incorporated herein in its entirety by reference thereto.
However, there exists a need to have an anti-drool mechanism that can be added as a modular component to existing injection molding systems, including stack molding systems.